Method and apparatus for transmitting reference signal in wireless communication system

ABSTRACT

A method for mapping, by a base station, a channel state information (CSI)-reference signal (RS) in a wireless communication system, includes mapping the CSI-RS, for at least one antenna port, to at least one pair of resource elements (REs) per physical resource block (PRB) pair in consecutive orthogonal frequency division multiplexing (OFDM) symbols in a subframe, wherein the subframe includes two slots, and each slot includes six OFDM symbols based on an extended cyclic prefix (CP), and transmitting the CSI-RS to a user equipment (UE) via the at least one antenna port in the subframe.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.15/402,887 filed on Jan. 10, 2017 (now U.S. Pat. No. 10,028,265 issuedon Jul. 17, 2018), which is a Continuation of U.S. patent applicationSer. No. 14/746,479 filed on Jun. 22, 2015 (now U.S. Pat. No. 9,572,137issued on Feb. 14, 2017), which is a Continuation of U.S. patentapplication Ser. No. 13/498,273 filed on Jun. 27, 2012 (now U.S. Pat.No. 9,083,482 issued on Jul. 14, 2015), which is filed as the NationalPhase of PCT/KR2010/006543 filed on Sep. 27, 2010, which claims thebenefit under 35 U.S.C. § 119(e) to U.S. Provisional Application Nos.61/361,435 filed on Jul. 5, 2010, 61/323,881 filed on Apr. 14, 2010 and61/246,154 filed on Sep. 27, 2009, and under 35 U.S.C. § 119(a) toKorean Patent Application No. 10-2010-0093216 filed on Sep. 27, 2010,all of which are hereby expressly incorporated by reference into thepresent application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for transmitting a referencesignal in a wireless communication system.

Description of the Related Art

The next-generation multimedia wireless communication systems which arerecently being actively researched are required to process and transmitvarious pieces of information, such as video and wireless data as wellas the initial voice-centered services. The 4th generation wirelesscommunication systems which are now being developed subsequently to the3rd generation wireless communication systems are aiming at supportinghigh-speed data service of downlink 1 Gbps (Gigabits per second) anduplink 500 Mbps (Megabits per second). The object of the wirelesscommunication system is to establish reliable communications between anumber of users irrespective of their positions and mobility. However, awireless channel has abnormal characteristics, such as path loss, noise,a fading phenomenon due to multi-path, Inter-Symbol Interference (ISI),and the Doppler Effect resulting from the mobility of a user equipment.A variety of techniques are being developed in order to overcome theabnormal characteristics of the wireless channel and to increase thereliability of wireless communication.

Technology for supporting reliable and high-speed data service includesOrthogonal Frequency Division Multiplexing (OFDM), Multiple InputMultiple Output (MIMO), and so on. An OFDM system is being consideredafter the 3rd generation system which is able to attenuate the ISIeffect with low complexity. The OFDM system converts symbols, receivedin series, into N (N is a natural number) parallel symbols and transmitsthem on respective separated N subcarriers. The subcarriers maintainorthogonality in the frequency domain. It is expected that the marketfor mobile communication will shift from the existing Code DivisionMultiple Access (CDMA) system to an OFDM-based system. MIMO technologycan be used to improve the efficiency of data transmission and receptionusing multiple transmission antennas and multiple reception antennas.MIMO technology includes spatial multiplexing, transmit diversity,beam-forming and the like. An MIMO channel matrix according to thenumber of reception antennas and the number of transmission antennas canbe decomposed into a number of independent channels. Each of theindependent channels is called a layer or stream. The number of layersis called a rank.

In wireless communication systems, it is necessary to estimate an uplinkchannel or a downlink channel for the purpose of the transmission andreception of data, the acquisition of system synchronization, and thefeedback of channel information. In wireless communication systemenvironments, fading is generated because of multi-path time latency. Aprocess of restoring a transmit signal by compensating for thedistortion of the signal resulting from a sudden change in theenvironment due to such fading is referred to as channel estimation. Itis also necessary to measure the state of a channel for a cell to whicha user equipment belongs or other cells. To estimate a channel ormeasure the state of a channel, a Reference Signal (RS) which is knownto both a transmitter and a receiver can be used.

A subcarrier used to transmit the reference signal is referred to as areference signal subcarrier, and a subcarrier used to transmit data isreferred to as a data subcarrier. In an OFDM system, a method ofassigning the reference signal includes a method of assigning thereference signal to all the subcarriers and a method of assigning thereference signal between data subcarriers. The method of assigning thereference signal to all the subcarriers is performed using a signalincluding only the reference signal, such as a preamble signal, in orderto obtain the throughput of channel estimation. If this method is used,the performance of channel estimation can be improved as compared withthe method of assigning the reference signal between data subcarriersbecause the density of reference signals is in general high. However,since the amount of transmitted data is small in the method of assigningthe reference signal to all the subcarriers, the method of assigning thereference signal between data subcarriers is used in order to increasethe amount of transmitted data. If the method of assigning the referencesignal between data subcarriers is used, the performance of channelestimation can be deteriorated because the density of reference signalsis low. Accordingly, the reference signals should be properly arrangedin order to minimize such deterioration.

A receiver can estimate a channel by separating information about areference signal from a received signal because it knows the informationabout a reference signal and can accurately estimate data, transmittedby a transmit stage, by compensating for an estimated channel value.Assuming that the reference signal transmitted by the transmitter is p,channel information experienced by the reference signal duringtransmission is h, thermal noise occurring in the receiver is n, and thesignal received by the receiver is y, it can result in y=h·p+n. Here,since the receiver already knows the reference signal p, it can estimatea channel information value ĥ using Equation 1 in the case in which aLeast Square (LS) method is used.ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Equation 1]

The accuracy of the channel estimation value ĥ estimated using thereference signal p is determined by the value {circumflex over (n)}. Toaccurately estimate the value h, the value {circumflex over (n)} mustconverge on 0. To this end, the influence of the value {circumflex over(n)} has to be minimized by estimating a channel using a large number ofreference signals. A variety of algorithms for a better channelestimation performance may exist.

There is a channel state information (CSI)-reference signal (RS) forchannel estimation in 3rd generation partnership project (3GPP) longterm evolution-advanced (LTE-A). The CSI-RS can be transmitted for eachof a plurality of layers. Meanwhile, a subframe having an extendedcyclic prefix (CP) structure has a small number of available OFDMsymbols, and thus has a small number of OFDM symbols to which the CSI-RScan be mapped. Accordingly, collision may occur when the CSI-RS istransmitted in a plurality of cells. Therefore, there is a need for aCSI-RS transmission method for improving channel estimation performancein the subframe having the extended CP structure.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for transmitting areference signal in a wireless communication system.

In an aspect, a method for transmitting a reference signal (RS) by abase station in a wireless communication system is provided. The methodincludes generating a channel state information (CSI)-RS for each of aplurality of layers, mapping the plurality of CSI-RSs to a resourceelement set consisting of a plurality of resource elements (REs) in asubframe, and transmitting the subframe to which the plurality ofCSI-RSs are mapped, wherein the plurality of REs constituting theresource element set are resource elements that are arranged withconstant subcarrier spacing in two neighboring orthogonal frequencydivision multiplexing (OFDM) symbols, and wherein the subframe has anextended cyclic prefix (CP) structure including 12 OFDM symbols. Theresource element included in the resource element set may be a resourceelement to which a cell-specific reference signal (CRS) of 3rdgeneration partnership project (3GPP) long-term evolution (LTE) rel-8and a user equipment (UE)-specific RS of 3GPP LTE-advanced (LTE-A) arenot mapped, and wherein the CRS of the 3GPP LTE rel-8 may be a CRS forantenna ports 0 and 1. The resource element set may be any one selectedfrom a plurality of different resource element sets. The number of thedifferent resource element sets may be 7. The resource element set maybe any one selected from the plurality of different resource elementsets on the basis of a cell identifier (ID). A CSI-RS mapped to the sameresource element in the resource element set among the plurality ofCSI-RSs may be multiplexed by using code division multiplexing (CDM)along a time domain. The CSI-RS may be CDM-multiplexed on the basis ofan orthogonal sequence having a length of 2. The two neighboring OFDMsymbols may be 5th and 6th OFDM symbols or 8th and 9th OFDM symbols or11th and 12th OFDM symbols. The constant subcarrier spacing may be3-subcarrier spacing.

In another aspect, a method for channel estimation by a user equipmentin a wireless communication system is provided. The method includesreceiving a channel state information (CSI) reference signal (RS) foreach of a plurality of layers, and estimating a channel by processingthe plurality of received CSI-RSs, wherein the plurality of CSI-RSs aretransmitted by being mapped to a resource element set consisting of aplurality of REs in a subframe, wherein the plurality of resourceelements constituting the resource element set are resource elementsthat are arranged with 3-subcarrier spacing in two neighboring OFDMsymbols, and wherein the subframe has an extended CP structure including12 OFDM symbols.

In another aspect, an apparatus for transmitting a reference signal (RS)is provided. The apparatus includes a radio frequency (RF) unit fortransmitting a subframe, and a processor coupled to the RF unit, whereinthe processor is configured for generating a CSI-RS for each of aplurality of layers, and mapping the plurality of CSI-RSs to a resourceelement set consisting of a plurality of REs in the subframe, whereinthe plurality of resource elements constituting the resource element setare resource elements that are arranged with 3-subcarrier spacing in twoneighboring OFDM symbols, and wherein the subframe has an extended CPstructure including 12 OFDM symbols.

According to the present invention, channel estimation performance canbe improved by increasing the number of resources elements to which achannel state information (CSI)-reference signal (RS) of 3rd generationpartnership project (3GPP) long term evolution-advanced (LTE-A) can bemapped.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows the structure of a radio frame in 3GPP LTE.

FIG. 3 shows a structure of a time division duplex (TDD) radio frame in3GPP LTE.

FIG. 4 shows an example of a resource grid of a single downlink slot.

FIG. 5 shows the structure of a downlink subframe.

FIG. 6 shows the structure of an uplink subframe.

FIGS. 7 to 9 show an exemplary CRS structure.

FIGS. 10 and 11 show examples of a DRS structure.

FIG. 12 shows an example of a UE-specific RS structure in 3GPP LTE-A.

FIGS. 13 to 16 show an example of a CSI-RS pattern according to theproposed invention.

FIG. 17 shows the proposed RS transmission method according to anembodiment of the present invention.

FIG. 18 shows the proposed channel estimation method according to anembodiment of the present invention.

FIG. 19 is a block diagram showing a BS and a UE according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following technique may be used for various wireless communicationsystems such as code division multiple access (CDMA), a frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), singlecarrier-frequency division multiple access (SC-FDMA), and the like. TheCDMA may be implemented as a radio technology such as universalterrestrial radio access (UTRA) or CDMA2000. The TDMA may be implementedas a radio technology such as a global system for mobile communications(GSM)/general packet radio service (GPRS)/enhanced data rates for GSMevolution (EDGE). The OFDMA may be implemented by a radio technologysuch as IEEE (Institute of Electrical and Electronics Engineers) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (Evolved UTRA), andthe like. IEEE 802.16m, an evolution of IEEE 802.16e, provides backwardcompatibility with a system based on IEEE 802.16e. The UTRA is part of auniversal mobile telecommunications system (UMTS). 3GPP (3rd generationpartnership project) LTE (long term evolution) is part of an evolvedUMTS (E-UMTS) using the E-UTRA, which employs the OFDMA in downlink andthe SC-FDMA in uplink. LTE-A (advanced) is an evolution of 3GPP LTE.

Hereinafter, for clarification, LET-A will be largely described, but thetechnical concept of the present invention is not meant to be limitedthereto.

FIG. 1 shows a wireless communication system.

The wireless communication system 10 includes at least one base station(BS) 11. Respective BSs 11 provide a communication service to particulargeographical areas 15 a, 15 b, and 15 c (which are generally calledcells). Each cell may be divided into a plurality of areas (which arecalled sectors). A user equipment (UE) 12 may be fixed or mobile and maybe referred to by other names such as MS (mobile station), MT (mobileterminal), UT (user terminal), SS (subscriber station), wireless device,PDA (personal digital assistant), wireless modem, handheld device. TheBS 11 generally refers to a fixed station that communicates with the UE12 and may be called by other names such as eNB (evolved-NodeB), BTS(base transceiver system), access point (AP), etc.

In general, a UE belongs to one cell, and the cell to which a UE belongsis called a serving cell. A BS providing a communication service to theserving cell is called a serving BS. The wireless communication systemis a cellular system, so a different cell adjacent to the serving cellexists. The different cell adjacent to the serving cell is called aneighbor cell. A BS providing a communication service to the neighborcell is called a neighbor BS. The serving cell and the neighbor cell arerelatively determined based on a UE.

This technique can be used for downlink or uplink. In general, downlinkrefers to communication from the BS 11 to the UE 12, and uplink refersto communication from the UE 12 to the BS 11. In downlink, a transmittermay be part of the BS 11 and a receiver may be part of the UE 12. Inuplink, a transmitter may be part of the UE 12 and a receiver may bepart of the BS 11.

FIG. 2 shows the structure of a radio frame in 3GPP LTE. It may bereferred to Paragraph 4.1 of “Technical Specification Group Radio AccessNetwork; Evolved Universal Terrestrial Radio Access (E-UTRA); Physicalchannels and modulation (Release 8)” to 3GPP (3rd generation partnershipproject) TS 36.211 V8.2.0 (March 2008).

Referring to FIG. 2, the radio frame includes 10 subframes, and onesubframe includes two slots. The slots in the radio frame are numberedby #0 to #19. A time taken for transmitting one subframe is called atransmission time interval (TTI). The TTI may be a scheduling unit for adata transmission. For example, a radio frame may have a length of 10ms, a subframe may have a length of 1 ms, and a slot may have a lengthof 0.5 ms.

One slot includes a plurality of OFDM (Orthogonal Frequency DivisionMultiplexing) symbols in a time domain and a plurality of subcarriers ina frequency domain. Since 3GPP LTE uses OFDMA in downlink, the OFDMsymbols are used to express a symbol period. The OFDM symbols may becalled by other names depending on a multiple-access scheme. Forexample, when SC-FDMA is in use as an uplink multi-access scheme, theOFDM symbols may be called SC-FDMA symbols. A resource block (RB), aresource allocation unit, includes a plurality of continuous subcarriersin a slot. The structure of the radio frame is merely an example.Namely, the number of subframes included in a radio frame, the number ofslots included in a subframe, or the number of OFDM symbols included ina slot may vary. 3GPP LTE defines that one slot includes seven OFDMsymbols in a normal cyclic prefix (CP) and one slot includes six OFDMsymbols in an extended CP.

FIG. 3 shows a structure of a time division duplex (TDD) radio frame in3GPP LTE. Section 4.2 of the 3GPP TS 36.211 V8.2.0 (March 2008)“Technical Specification Group Radio Access Network; Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 8)” may be incorporated herein by reference. One radio framehas a length of 10 milliseconds (ms) and consists of two half-frameseach having a length of 5 ms. One half-frame consists of five subframeseach having a length of 1 ms.

One subframe is designated as any one of an uplink (UL) subframe, adownlink (DL) subframe, and a special subframe. Table 1 shows astructure of a configurable frame according to arrangement of the ULsubframe and the DL subframe in a 3GPP LTE TDD system. In Table 1, ‘D’denotes the DL subframe, ‘U’ denotes the UL subframe, and ‘S’ denotesthe special subframe.

TABLE 1 Uplink- Downlink- downlink to-Uplink config- Switch-pointSubframe number uration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U UD S U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D DD D D 6 5 ms D S U U U D S U U D

The switch-point periodicity may be 5 ms or 10 ms. In case of the 5 msswitch-point periodicity, the special subframe may be present in both oftwo half-frames in one subframe. In case of the 10 ms switch-pointperiodicity, the special subframe may be present only in a firsthalf-frame.

The special subframe is a specific period positioned between the ULsubframe and the DL subframe for the purpose of UL-DL separation. Oneradio frame includes at least one special subframe. The special subframeincludes a downlink pilot time slot (DwPTS), a guard period (GP), and anuplink pilot time slot (UpPTS). The DwPTS is used for initial cellsearch, synchronization, or channel estimation. The UpPTS is used forchannel estimation in a BS and UL transmission synchronization of a UE.The GP is positioned between the UL time slot and the DL time slot andis used to remove interference that occurs in UL transmission due to amulti-path delay of a DL signal. Table 2 shows lengths of the DwPTS, theGP, and the UpPTS according to a structure of the special subframe.

TABLE 2 Normal cyclic prefix in downlink Extended cyclic prefix indownlink UpPTS UpPTS Special Normal Extended Normal Extended subframecyclic prefix cyclic prefix cyclic prefix cyclic prefix configurationDwPTS in uplink in uplink DwPTS in uplink in uplink 0  6592*Ts 2192*Ts2560*Ts  7680*Ts 2192*Ts 2560*Ts 1 19760*Ts 20480*Ts 2 21952*Ts 23040*Ts3 24144*Ts 25600*Ts 4 26336*Ts  7680*Ts 4384*Ts 5120*ts 5  6592*Ts4384*Ts 5120*ts 20480*Ts 6 19760*Ts 23040*Ts 7 21952*Ts — 8 24144*Ts —

The subframe 0, the subframe 5, and the DwPTS of the special subframeare always allocated for downlink transmission. The UpPTS of the specialsubframe and the subframe following the special subframe are alwaysallocated for uplink transmission.

FIG. 4 shows an example of a resource grid of a single downlink slot.

A downlink slot includes a plurality of OFDM symbols in the time domainand NRB number of resource blocks (RBs) in the frequency domain. The NRBnumber of resource blocks included in the downlink slot is dependentupon a downlink transmission bandwidth set in a cell. For example, in anLTE system, NRB may be any one of 60 to 110. One resource block includesa plurality of subcarriers in the frequency domain. An uplink slot mayhave the same structure as that of the downlink slot.

Each element on the resource grid is called a resource element. Theresource elements on the resource grid can be discriminated by a pair ofindexes (k,l) in the slot. Here, k (k=0, . . . , NRB×12−1) is asubcarrier index in the frequency domain, and l is an OFDM symbol indexin the time domain.

Here, it is illustrated that one resource block includes 7×12 resourceelements made up of seven OFDM symbols in the time domain and twelvesubcarriers in the frequency domain, but the number of OFDM symbols andthe number of subcarriers in the resource block are not limited thereto.The number of OFDM symbols and the number of subcarriers may varydepending on the length of a cyclic prefix (CP), frequency spacing, andthe like. For example, in case of a normal CP, the number of OFDMsymbols is 7, and in case of an extended CP, the number of OFDM symbolsis 6. One of 128, 256, 512, 1024, 1536, and 2048 may be selectively usedas the number of subcarriers in one OFDM symbol.

FIG. 5 shows the structure of a downlink subframe.

A downlink subframe includes two slots in the time domain, and each ofthe slots includes seven OFDM symbols in the normal CP. First three OFDMsymbols (maximum four OFDM symbols with respect to a 1.4 Mhz bandwidth)of a first slot in the subframe corresponds to a control region to whichcontrol channels are allocated, and the other remaining OFDM symbolscorrespond to a data region to which a physical downlink shared channel(PDSCH) is allocated.

The PDCCH may carry a transmission format and a resource allocation of adownlink shared channel (DL-SCH), resource allocation information of anuplink shared channel (UL-SCH), paging information on a PCH, systeminformation on a DL-SCH, a resource allocation of an higher layercontrol message such as a random access response transmitted via aPDSCH, a set of transmission power control commands with respect toindividual UEs in a certain UE group, an activation of a voice overinternet protocol (VoIP), and the like. A plurality of PDCCHs may betransmitted in the control region, and a UE can monitor a plurality ofPDCCHs. The PDCCHs are transmitted on one or an aggregation of aplurality of consecutive control channel elements (CCE). The CCE is alogical allocation unit used to provide a coding rate according to thestate of a wireless channel. The CCD corresponds to a plurality ofresource element groups. The format of the PDCCH and an available numberof bits of the PDCCH are determined according to an associative relationbetween the number of the CCEs and a coding rate provided by the CCEs.

The BS determines a PDCCH format according to a DCI to be transmitted tothe UE, and attaches a cyclic redundancy check (CRC) to the DCI. Aunique radio network temporary identifier (RNTI) is masked on the CRCaccording to the owner or the purpose of the PDCCH. In case of a PDCCHfor a particular UE, a unique identifier, e.g., a cell-RNTI (C-RNTI), ofthe UE, may be masked on the CRC. Or, in case of a PDCCH for a pagingmessage, a paging indication identifier, e.g., a paging-RNTI (P-RNTI),may be masked on the CRC. In case of a PDCCH for a system informationblock (SIB), a system information identifier, e.g., a systeminformation-RNTI (SI-RNTI), may be masked on the CRC. In order toindicate a random access response, i.e., a response to a transmission ofa random access preamble of the UE, a random access-RNTI (RA-RNTI) maybe masked on the CRC.

FIG. 6 shows the structure of an uplink subframe.

An uplink subframe may be divided into a control region and a dataregion in the frequency domain. A physical uplink control channel(PUCCH) for transmitting uplink control information is allocated to thecontrol region. A physical uplink shared channel (PUCCH) fortransmitting data is allocated to the data region. When indicated by ahigher layer, the UE may support a simultaneous transmission of thePUSCH and the PUCCH.

The PUCCH with respect to a UE is allocated by a pair of resource blocksin a subframe. The resource blocks belonging to the pair of resourceblocks (RBs) occupy different subcarriers in first and second slots,respectively. The frequency occupied by the RBs belonging to the pair ofRBs is changed based on a slot boundary. This is said that the pair ofRBs allocated to the PUCCH are frequency-hopped at the slot boundary.The UE can obtain a frequency diversity gain by transmitting uplinkcontrol information through different subcarriers according to time. InFIG. 6, m is a position index indicating the logical frequency domainpositions of the pair of RBs allocated to the PUCCH in the subframe.

Uplink control information transmitted on the PUCCH may include a hybridautomatic repeat request (HARQ) acknowledgement/non-acknowledgement(ACK/NACK), a channel quality indicator (CQI) indicating the state of adownlink channel, an scheduling request (SR), and the like.

The PUSCH is mapped to a uplink shared channel (UL-SCH), a transportchannel. Uplink data transmitted on the PUSCH may be a transport block,a data block for the UL-SCH transmitted during the TTI. The transportblock may be user information. Or, the uplink data may be multiplexeddata. The multiplexed data may be data obtained by multiplexing thetransport block for the UL-SCH and control information. For example,control information multiplexed to data may include a CQI, a precodingmatrix indicator (PMI), an HARQ, a rank indicator (RI), or the like. Orthe uplink data may include only control information.

A reference signal is generally transmitted as a sequence. A referencesignal sequence is not particularly limited and a certain sequence maybe used as the reference signal sequence. As the reference signalsequence, a sequence generated through a computer based on phase shiftkeying (PSK) (i.e., a PSK-based computer generated sequence) may beused. The PSK may include, for example, binary phase shift keying(BPSK), quadrature phase shift keying (QPSK), and the like. Or, as thereference signal sequence, a constant amplitude zero auto-correlation(CAZAC) may be used. The CAZAC sequence may include, for example, aZadoff-Chu (ZC)-based sequence, a ZC sequence with cyclic extension, aZC sequence with truncation, and the like. Also, as the reference signalsequence, a pseudo-random (PN) sequence may be used. The PN sequence mayinclude, for example, an m-sequence, a sequence generated through acomputer, a gold sequence, a Kasami sequence, and the like. Also, acyclically shifted sequence may be used as the reference signalsequence.

A reference signal can be classified into a cell-specific referencesignal (CRS), an MBSFN reference signal, a user equipment-specificreference signal (UE-specific RS) and a position reference signal (PRS).The CRS is transmitted to all the UEs within a cell and used for channelestimation. The MBSFN reference signal can be transmitted in sub-framesallocated for MBSFN transmission. The UE-specific reference signal isreceived by a specific UE or a specific UE group within a cell, and maybe referred to a dedicated RS (DRS). The DRS is chiefly used by aspecific UE or a specific UE group for the purpose of data demodulation.

First, a CRS is described.

FIGS. 7 to 9 show an exemplary CRS structure. FIG. 7 shows an exemplaryCRS structure when a BS uses one antenna. FIG. 8 shows an exemplary CRSstructure when a BS uses two antennas. FIG. 9 shows an exemplary CRSstructure when a BS uses four antennas. The section 6.10.1 of 3GPP TS36.211 V8.2.0 (March 2008) may be incorporated herein by reference. Inaddition, the exemplary CRS structure may be used to support a featureof an LTE-A system. Examples of the feature of the LTE-A system includecoordinated multi-point (CoMP) transmission and reception, spatialmultiplexing, etc.

Referring to FIG. 7 to FIG. 9, in multi-antenna transmission, a BS usesa plurality of antennas, each of which has one resource grid. ‘R0’denotes an RS for a first antenna, ‘R1’ denotes an RS for a secondantenna, ‘R2’ denotes an RS for a third antenna, and ‘R3’ denotes an RSfor a fourth antenna. R0 to R3 are located in a subframe withoutoverlapping with one another.

indicates a position of an OFDM symbol in a slot. In case of a normalcyclic prefix (CP),

has a value in the range of 0 to 6. In one OFDM symbol, RSs for therespective antennas are located with a spacing of 6 subcarriers. In asubframe, the number of R0s is equal to the number of R1s, and thenumber of R2s is equal to the number of R3s. In the subframe, the numberof R2s and R3s is less than the number of R0s and R1s. A resourceelement used for an RS of one antenna is not used for an RS of anotherantenna. This is to avoid interference between antennas.

The CRS is always transmitted by the number of antennas irrespective ofthe number of streams. The CRS has an independent RS for each antenna. Afrequency-domain position and a time-domain position of the CRS in asubframe are determined irrespective of a UE. A CRS sequence to bemultiplied to the CRS is generated also irrespective of the UE.Therefore, all UEs in a cell can receive the CRS. However, a position ofthe CRS in the subframe and the CRS sequence may be determined accordingto a cell identifier (ID). The time-domain position of the CRS in thesubframe may be determined according to an antenna number and the numberof OFDM symbols in a resource block. The frequency-domain position ofthe CRS in the subframe may be determined according to an antennanumber, a cell ID, an OFDM symbol index

, a slot number in a radio frame, etc.

The CRS sequence may be applied on an OFDM symbol basis in one subframe.The CRS sequence may differ according to a cell ID, a slot number in oneradio frame, an OFDM symbol index in a slot, a CP type, etc. The numberof RS subcarriers for each antenna on one OFDM symbol is 2. When asubframe includes NRB resource blocks in a frequency domain, the numberof RS subcarriers for each antenna on one OFDM symbol is 2×NRB.Therefore, a length of the CRS sequence is 2×NRB.

Equation 2 shows an example of a CRS sequence r(m).

$\begin{matrix}{{r(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\;\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2m} + 1} \right)}}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Herein, m is 0, 1, . . . , 2NRB,max−1. NRB,max denotes the number ofresource blocks corresponding to a maximum bandwidth. For example, whenusing a 3GPP LTE system, NRB,max is 110. c(i) denotes a PN sequence as apseudo-random sequence, and can be defined by a gold sequence having alength of 31. Equation 3 shows an example of a gold sequence c(n).c(n)=(x ₁(n+N _(C))+x ₂(n+N _(C)))mod 2x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2  [Equation 3]

Herein, NC is 1600, x1(i) denotes a 1st m-sequence, and x2(i) denotes a2nd m-sequence. For example, the 1st m-sequence or the 2nd m-sequencecan be initialized for each OFDM symbol according to a cell ID, a slotnumber in one radio frame, an OFDM symbol index in a slot, a CP type,etc.

In case of using a system having a bandwidth narrower than NRB,max, acertain part with a length of 2×NRB can be selected from an RS sequencegenerated in a length of 2×NRB,max.

The CRS may be used in the LTE-A system to estimate channel stateinformation (CSI). A reference signal for estimating channel stateinformation may be referred to a channel state information referencesignal (CSI-RS). A CSI-RS is relatively sparse deployed in a frequencydomain or a time domain. A CSI-RS may be punctured in a data region of anormal subframe or an MBSFN subframe. If necessary for estimation of theCSI, channel quality indicator (CQI), a precoding matrix indicator(PMI), a rank indicator (RI), or the like may be reported from the UE.

A DRS is described below.

FIGS. 10 and 11 show examples of a DRS structure. FIG. 10 shows anexample of the DRS structure in the normal CP (Cyclic Prefix). In thenormal CP, a subframe includes 14 OFDM symbols. R5 indicates thereference signal of an antenna which transmits a DRS. On one OFDM symbolincluding a reference symbol, a reference signal subcarrier ispositioned at intervals of four subcarriers. FIG. 11 shows an example ofthe DRS structure in the extended CP. In the extended CP, a subframeincludes 12 OFDM symbols. On one OFDM symbol, a reference signalsubcarrier is positioned at intervals of three subcarriers. For detailedinformation, reference can be made to Paragraph 6.10.3 of 3GPP TS 36.211V8.2.0 (March 2008).

The position of a frequency domain and the position of a time domainwithin the subframe of a DRS can be determined by a resource blockassigned for PDSCH transmission. A DRS sequence can be determined by aUE ID, and only a specific UE corresponding to the UE ID can receive aDRS.

A DRS sequence can be obtained using Equations 2 and 3. However, m inEquation 2 is determined by NRBPDSCH. NRBPDSCH is the number of resourceblocks corresponding to a bandwidth corresponding to PDSCH transmission.The length of a DRS sequence can be changed according to NRBPDSCH. Thatis, the length of a DRS sequence can be changed according to the amountof data assigned to a UE. In Equation 2, a first m-sequence x1(i) or asecond m-sequence x2(i) can be reset according to a cell ID, theposition of a subframe within one radio frame, a UE ID, etc. for everysubframe.

A DRS sequence can be generated for every subframe and applied for everyOFDM symbol. It is assumed that the number of reference signalsubcarriers per resource block is 12 and the number of resource blocksis NRBPDSCH, within one subframe. The total number of reference signalsubcarriers is 12×NRBPDSCH. Accordingly, the length of the DRS sequenceis 12×NRBPDSCH. In the case in which DRS sequences are generated usingEquation 2, m is 0, 1, . . . , 12NRBPDSCH−1. The DRS sequences aresequentially mapped to reference symbols. The DRS sequence is firstmapped to the reference symbol and then to a next OFDM symbol, inascending powers of a subcarrier index in one OFDM symbol.

Further, a Cell-specific Reference Signal (CRS) can be used togetherwith a DRS. For example, it is assumed that control information istransmitted through three OFDM symbols (l=0, 1, 2) of a first slotwithin a subframe. A CRS can be used in an OFDM symbol having an indexof 0, 1, or 2 (l=0, 1, or 2), and a DRS can be used in the remainingOFDM symbol other than the three OFDM symbols. Here, by transmitting apredefined sequence which is multiplied by a downlink reference signalfor each cell, interference between reference signals received by areceiver from neighbor cells can be reduced, and so the performance ofchannel estimation can be improved. The predefined sequence can be oneof a PN sequence, an m-sequence, a Walsh hadamard sequence, a ZCsequence, a GCL sequence, and a CAZAC sequence. The predefined sequencecan be applied to each OFDM symbol within one subframe, and anothersequence can be applied depending on a cell ID, a subframe number, theposition of an OFDM symbol, and a UE ID.

In the LTE-A system, a DRS can be use in PDSCH demodulation. Here, aPDSCH and a DRS can comply with the same precoding operation. The DRScan be transmitted only in a resource block or layer scheduled by a BaseStation (BS), and orthogonality is maintained between layers.

FIG. 12 shows an example of a UE-specific RS structure in 3GPP LTE-A.

Referring to FIG. 12, a UE-specific RS for two antenna ports R7 and R8is transmitted in a subframe having an extended CP structure. In the3GPP LTE-A, the UE-specific RS can support up to two antenna ports. AUE-specific RS for the antenna port 7 is transmitted on a resourceelement corresponding to 2nd, 5th, 8th, and 11th subcarriers of 5th and6th OFDM symbols and a resource element corresponding to 1st, 4th, 7th,and 10th subcarriers of 11th and 12th OFDM symbols. Likewise, aUE-specific RS for the antenna port 8 is transmitted on a resourceelement corresponding to 2nd, 5th, 8th, and 11th subcarriers of 5th and6th OFDM symbols and a resource element corresponding to 1st, 4th, 7th,and 10th subcarriers of 11th and 12th OFDM symbols. That is, theUE-specific RSs for the antenna ports 7 and 8 can be transmitted bybeing mapped to the same resource element. In this case, an RS sequencefor each antenna port can be transmitted by being multiplied bydifferent orthogonal sequences. For example, an orthogonal sequence [+1+1] having a length of 2 and mapped to neighboring OFDM symbols can bemultiplied by an RS sequence for the antenna port 7. In addition, anorthogonal sequence [−1 +1] having a length of 2 and mapped toneighboring OFDM symbols can be multiplied by an RS sequence for theantenna port 8. Examples of the orthogonal sequence may include varioustypes of orthogonal sequences, such as, a Walsh code, a discrete Fouriertransform (DFT) coefficient, a CAZAC sequence, etc.

In addition to a CRS of 3GPP LTE rel-8 and a UE-specific RS for eachantenna port, a CSI-RS for 3GPP LTE-A used to estimate or measure achannel state can be additionally transmitted by being mapped to aresource block. The CSI-RS can be mapped while avoiding overlapping withthe resource element to which the CRS or the UE-specific RS is mapped.The CSI-RS can be transmitted for each antenna port.

Meanwhile, a CRS for the antenna ports 2 and 3 may not be transmitted inthe subframe having the extended CP structure. That is, the CRSstructure of FIG. 8 may be used rather than the CRS structure of FIG. 9.This is to effectively utilize insufficient radio resources since thesubframe having the extended CP structure consists of 12 OFDM symbols.Accordingly, an OFDM symbol on which the CRS for the antenna ports 2 and3 is transmitted can be allocated for the CSI-RS or another RS, or canbe allocated to data.

Hereinafter, the proposed RS transmission method will be describedaccording to an embodiment of the present invention. The presentinvention proposes various RS patterns for mapping a CSI-RS when an RSof the antenna ports 2 and 3 of 3GPP LTE rel-8 is not transmitted.Although a subframe having an extended CP structure is described forexample in the present invention, the present invention can also equallyapply to a subframe having a normal CP structure. In an RS patternproposed in the present invention, a horizontal axis represents a timedomain or an OFDM symbol (indexed with OFDM symbol indices 0 to 11), anda vertical axis represents a frequency domain or a subcarrier (indexedwith subcarrier indices 0 to 11). In addition, R0 and R1 represent a CRSof 3GPP LTE rel-8 for antenna ports 0 and 1. The R0 and R1 are mappedaccording to the CRS structure of FIG. 8. D1 to D4 represent UE-specificRSs (hereinafter, DRSs) of four antenna ports. The DRSs of the fourantenna ports can be grouped in pair of two, and then be transmitted bybeing CDM-multiplexed. The DRS can be transmitted by being mapped to aresource block according to the UE-specific RS structure of FIG. 12, orcan be transmitted by being mapped to a resource block according to apattern in which the UE-specific RS structure of FIG. 12 is shiftedalong a frequency axis.

FIG. 13 shows an example of a CSI-RS pattern according to the proposedinvention.

Referring to FIG. 13, a resource element to which a CRS and DRS of 3GPPLTE rel-8 are not mapped can be used as a resource element to which aCSI-RS can be mapped. To support up to 8 antenna ports, 8 resourceelements can be used in CSI-RS transmission. The 8 resource elements towhich the CSI-RS can be mapped constitute one resource element set, and5 resource element sets may be present in total. A 1st set may include aresource element corresponding to 1st, 4th, 7th, and 10th subcarriers of5th and 6th OFDM symbols which are OFDM symbols to which a DRS ismapped. A 2nd set to a 4th set occupy 8th and 9th OFDM symbols, andamong them, the 2nd set includes a resource element corresponding to1st, 4th, 7th, and 10th subcarriers, the 3rd set includes a resourceelement corresponding to 2nd, 5th, 8th, and 11th subcarriers, and the4th set includes a resource element corresponding to 3rd, 6th, 9th, and12th subcarriers. The 5th set may include a resource elementcorresponding to 3rd, 6th, 9th, and 12th subcarriers of 11th and 12thOFDM symbols which are OFDM symbols to which a DRS is mapped. A CSI-RSfor up to 8 antenna ports is transmitted in a resource elementconstituting each resource element set. A location of a resource elementto which a CSI-RS of each antenna port is mapped may change in eachresource element set. Further, the CSI-RS can be transmitted by beingmapped to a different resource element set for each subframe. A BS canselect any one set among the 1st set to the 5th set and can map theCSI-RS to a resource element that constitutes a corresponding resourceelement set. In this case, the selected set may be selected based on acell ID, for example, based on the equation of (cell ID mod 5)+1.

FIG. 14 shows another example of a CSI-RS pattern according to theproposed invention.

Referring to FIG. 14, a CSI-RS is transmitted by being mapped to aresource element set configured in the same manner as FIG. 13, exceptthat the CSI-RS for 8 antenna ports are grouped in pair and areCDM-multiplexed along a time domain. Among resource elements included ina 1st set, a CSI-RS for 1st and 2nd antenna ports can be mapped to aresource element included in a 1st subcarrier. The CSI-RS for the 1stand 2nd antenna ports can be CDM-multiplexed along the time domain byusing an orthogonal sequence having a length of 2. Since the CSI-RS isCDM-multiplexed along the time domain, it can be said that the CSI-RS ismultiplexed in a CDM-time (T) manner. Likewise, among the resourceelements included in the 1st set, a CSI-RS for 3rd and 4th antenna portscan be CDM-multiplexed and mapped along the time domain to a resourceelement included in a 4th subcarrier. In addition, a CSI-RS for 5th and6th antenna ports and a CSI-RS for 7th and 8th antenna ports can beCDM-multiplexed and mapped along the time domain respectively to aresource element included in a 7th subcarrier and a resource elementincluded in a 10th subcarrier. A CSI-RS of antenna ports which areCDM-multiplexed in the same carrier is not limited to the presentembodiment, and thus can be multiplexed by configuring it with variouscombinations. A location of a resource element to which a CSI-RS of eachantenna port is mapped may change in each resource element set. Further,the CSI-RS can be transmitted by being mapped to a different resourceelement set for each subframe. A BS can select any one set among the 1stset to the 5th set and can map the CSI-RS to a resource element thatconstitutes a corresponding resource element set. In this case, theselected set may be selected based on a cell ID, for example, based onthe equation of (cell ID mod 5)+1.

Meanwhile, a UE-specific RS of up to rank-2 can be supported. That is,instead of transmitting a DRS for 4 antenna ports, only a DRS for twoantenna ports can be transmitted. In this case, a resource elementallocated for a UE-specific RS of layers 3 and 4 can be allocated to aCSI-RS, and thus the number of resource element sets to which the CSI-RScan be mapped may increase.

FIG. 15 shows another example of a CSI-RS pattern according to theproposed invention.

The CSI-RS pattern of FIG. 15 is different from the CSI-RS pattern ofFIG. 13 in a sense that a resource element allocated for a UE-specificRS of layers 3 and 4 is additionally allocated to 6th and 7th sets for aCSI-RS. The 6th set may include a resource element corresponding to 3rd,6th, 9th, and 12th subcarriers of 5th and 6th OFDM symbols which areOFDM symbols to which a DRS is mapped. The 7th set may include aresource element corresponding to 2nd, 5th, 8th, and 11th subcarriers of11th and 12th OFDM symbols which are OFDM symbols to which a DRS ismapped. Accordingly, the number of resource element sets that can beselected by a BS to transmit a CSI-RS increases, and channel estimationperformance can be improved by avoiding inter-cell interference (ICI)for a CSI-RS in a multi-cell environment. A CSI-RS for up to 8 antennaports is transmitted in a resource element constituting each resourceelement set. A location of a resource element to which a CSI-RS of eachantenna port is mapped may change in each resource element set. Further,the CSI-RS can be transmitted by being mapped to a different resourceelement set for each subframe. The BS can select any one set among the1st set to the 7th set and can map the CSI-RS to a resource element thatconstitutes a corresponding resource element set. In this case, theselected set may be selected based on a cell ID, for example, based onthe equation of (cell ID mod 7)+1.

FIG. 16 shows another example of a CSI-RS pattern according to theproposed invention. Referring to FIG. 16, a CSI-RS is transmitted bybeing mapped to a resource element set configured in the same manner asFIG. 15, except that the CSI-RS for 8 antenna ports are grouped in pairand are CDM-multiplexed along a time domain. Among resource elementsincluded in a 1st set, a CSI-RS for 1st and 2nd antenna ports can bemapped to a resource element included in a 1st subcarrier. A CSI-RS forthe 1st and 2nd antenna ports can be CDM-multiplexed along the timedomain by using an orthogonal sequence having a length of 2. Since theCSI-RS is CDM-multiplexed along the time domain, it can be said that theCSI-RS is multiplexed in a CDM-T manner. Likewise, among the resourceelements included in the 1st set, a CSI-RS for 3rd and 4th antenna portscan be CDM-multiplexed and mapped along the time domain to a resourceelement included in a 4th subcarrier. In addition, a CSI-RS for 5th and6th antenna ports and a CSI-RS for 7th and 8th antenna ports can beCDM-multiplexed and mapped along the time domain respectively to aresource element included in a 7th subcarrier and a resource elementincluded in a 10th subcarrier. A CSI-RS of antenna ports which areCDM-multiplexed in the same carrier is not limited to the presentembodiment, and thus can be multiplexed by configuring it with variouscombinations. A location of a resource element to which a CSI-RS of eachantenna port is mapped may change in each resource element set. Further,the CSI-RS can be transmitted by being mapped to a different resourceelement set for each subframe. A BS can select any one set among the 1stset to the 7th set and can map the CSI-RS to a resource element thatconstitutes a corresponding resource element set. In this case, theselected set may be selected based on a cell ID, for example, based onthe equation of (cell ID mod 7)+1.

FIG. 17 shows the proposed RS transmission method according to anembodiment of the present invention.

In step S100, a BS generates a plurality of CSI-RSs for each of aplurality of layers. In step S110, the BS maps the plurality of CSI-RSsto a resource block. A resource element in the resource block to whichthe plurality of CSI-RSs are mapped is a resource element to which a CRSof 3GPP LTE rel-8 and a UE-specific RS are not mapped, and the pluralityof CSI-RSs can be mapped by being multiplexed with CDM-T. In step S120,the BS transmits the resource block to which the CSI-RS is mapped.

FIG. 18 shows the proposed channel estimation method according to anembodiment of the present invention.

In step S200, a UE receives a plurality of RSs for each of a pluralityof layers from a BS. The plurality of RSs may be a CSI-RS of 3GPP LTE-A.In step S210, the UE performs channel estimation by processing theplurality of RSs. A resource element in the resource block to which theplurality of RSs are mapped is a resource element to which a CRS of 3GPPLTE rel-8 and a UE-specific RS are not mapped, and the plurality ofCSI-RSs can be mapped by being multiplexed with CDM-T.

FIG. 19 is a block diagram showing a BS and a UE according to anembodiment of the present invention.

A BS 800 includes a processor 810, a memory 820, and a radio frequency(RF) unit 830. The processor 810 implements the proposed functions,processes, and/or methods. The processor 810 generates a plurality ofCSI-RSs for each of a plurality of layers and maps the plurality ofCSI-RSs to a resource block. A resource element in the resource block towhich the plurality of CSI-RSs are mapped is a resource element to whicha CRS of 3GPP LTE rel-8 and a UE-specific RS are not mapped, and theplurality of CSI-RSs can be mapped by being multiplexed with CDM-T. Theplurality of CSI-RSs can be mapped according to the RS patterns of FIG.13 to FIG. 16. Layers of a radio interface protocol can be implementedby the processor 810. The memory 820 coupled to the processor 810 storesa variety of information for driving the processor 810. The RF unit 830coupled to the processor 810 transmits and/or receives a radio signal,and transmits a resource block to which the plurality of CSI-RSs aremapped.

A UE 900 includes a processor 910, a memory 920, and an RF unit 930. TheRF unit 930 coupled to the processor 910 transmits and/or receives aradio signal and receives a plurality of RSs. The plurality of RSs maybe a CSI-RS of 3GPP LTE-A. The processor 910 implements the proposedfunctions, processes, and/or methods. The processor 910 performs channelestimation by processing the plurality of RSs. Layers of a radiointerface protocol can be implemented by the processor 910. The memory920 coupled to the processor 910 stores a variety of information fordriving the processor 910.

The processor 910 may include an application-specific integrated circuit(ASIC), another chip set, a logical circuit, and/or a data processingunit. The RF unit 920 may include a baseband circuit for processingradio signals. In software implemented, the aforementioned methods canbe implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be performed bythe processor 910.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method for mapping, by a base station, achannel state information (CSI)-reference signal (RS) in a wirelesscommunication system, the method comprising: mapping the CSI-RS, for atleast one antenna port, to at least one pair of resource elements (REs)per physical resource block (PRB) pair in consecutive orthogonalfrequency division multiplexing (OFDM) symbols in a subframe, whereinthe subframe includes two slots, and each slot includes six OFDM symbolsbased on an extended cyclic prefix (CP); and transmitting the CSI-RS toa user equipment (UE) via the at least one antenna port in the subframe.2. The method of claim 1, wherein a number of the at least one antennaport is eight.
 3. The method of claim 2, wherein the CSI-RS for eightantenna ports is mapped to four pairs of REs per PRB pair in theconsecutive OFDM symbols.
 4. The method of claim 3, wherein the fourpairs of REs per PRB pair are separated from each other by a constantsubcarrier spacing in the consecutive OFDM symbols.
 5. The method ofclaim 4, wherein the constant subcarrier spacing is 3-subcarrierspacing.
 6. The method of claim 1, wherein the consecutive OFDM symbolsare 2nd and 3rd OFDM symbols in a second slot of the subframe.
 7. Themethod of claim 6, wherein a cell-specific reference signal (CRS) is nottransmitted via antenna ports 2 and 3 in the 2nd and 3rd OFDM symbols inthe second slot of the subframe.
 8. The method of claim 1, wherein theconsecutive OFDM symbols are 5th and 6th OFDM symbols in a second slotor 5th and 6th OFDM symbols in a first slot of the subframe.
 9. Themethod of claim 1, wherein the subframe is a downlink (DL) subframe in atime division duplex (TDD) frame.
 10. A base station (BS) in a wirelesscommunication system, the BS comprising: a memory; a transceiver; and aprocessor, coupled to the memory and the transceiver, that: maps achannel state information (CSI)-reference signal (RS), for at least oneantenna port, to at least one pair of resource elements (REs) perphysical resource block (PRB) pair in consecutive orthogonal frequencydivision multiplexing (OFDM) symbols in a subframe, wherein the subframeincludes two slots, and each slot includes six OFDM symbols based on anextended cyclic prefix (CP), and transmits the CSI-RS to a userequipment (UE) via the at least one antenna port in the subframe. 11.The BS of claim 10, wherein a number of the at least one antenna port iseight.
 12. The BS of claim 11, wherein the CSI-RS for eight antennaports is mapped to four pairs of REs per PRB pair in the consecutiveOFDM symbols.
 13. The BS of claim 12, wherein the four pairs of REs perPRB pair are separated from each other by a constant subcarrier spacingin the consecutive OFDM symbols.
 14. The BS of claim 13, wherein theconstant subcarrier spacing is 3-subcarrier spacing.
 15. The BS of claim10, wherein the consecutive OFDM symbols are 2nd and 3rd OFDM symbols ina second slot of the subframe.
 16. The BS of claim 15, wherein acell-specific reference signal (CRS) is not transmitted via antennaports 2 and 3 in the 2nd and 3rd OFDM symbols in the second slot of thesubframe.
 17. The BS of claim 10, wherein the consecutive OFDM symbolsare 5th and 6th OFDM symbols in a second slot or 5th and 6th OFDMsymbols in a first slot of the subframe.
 18. The BS of claim 10, whereinthe subframe is a downlink (DL) subframe in a time division duplex (TDD)frame.